Scanning tunneling microscopy study of poly-L-proline

Area of Science:

• Biochemistry
• Materials Science
• Structural Biology

Background:

• Poly-L-proline, an imino acid polymer, exhibits two distinct geometric structures based on peptide bond conformation.
• Cis/trans isomerization of proline imide bonds is implicated in protein denaturation and folding.
• Understanding these conformations is crucial for protein structure-function relationships.

Purpose of the Study:

• To visualize and characterize the distinct geometric structures of poly-L-proline using high-resolution imaging.
• To investigate the conformational flexibility and aggregation behavior of poly-L-proline forms.
• To experimentally validate theoretical predictions regarding poly-L-proline conformations.

Main Methods:

• High-resolution imaging of air-dried poly-L-proline using scanning tunneling microscopy (STM).
• Utilizing the inherent electric conductivity of a poly-L-proline monolayer for STM analysis.
• Analyzing STM images to discern molecular rigidity, flexibility, and aggregation patterns.

Main Results:

• STM imaging confirmed the existence of two poly-L-proline forms with distinct structural properties.
• Form II (trans peptide bonds) was observed to be relatively rigid and prone to aggregation.
• Form I (cis peptide bonds) demonstrated significant flexibility, with sharp bends and backfolding.
• Sufficient conductivity of a single poly-L-proline layer enabled STM imaging with minimal tip-sample interaction.

Conclusions:

• The study provides direct visual evidence for the two predicted peptide bond conformations of poly-L-proline.
• Distinct structural behaviors of Form I and Form II poly-L-proline were experimentally confirmed.
• STM is a viable technique for imaging conductive biological macromolecules like poly-L-proline.